The present invention generally relates to solenoid valves and more particularly to an improved valve which varies the rate at which hydrocarbons can be removed from an evaporation canister.
An evaporation canister forms a part of an emission control system of an internal combustion engine. When the engine is turned off hydrocarbons in the intake manifold of the engine and the fuel tank are communicated to the evaporation canister which contains a filtering element. Upon starting the engine the free hydrocarbons within the evaporation canister are returned to the intake manifold through a return tube where they are combusted. In order to prevent an excessive amount of hydrocarbons from appearing in the exhaust when the engine is initially started the rate which the hydrocarbons are permitted to flow from the canister to the intake manifold must be controlled. Prior evaporation canister systems have included orifices or vacuum devices in the return line to control the rate at which the hydrocarbons enter the intake manifold. Vacuum devices are deficient for controlling the flow rate in that they often provide only an on/off control The present invention provides an improved solenoid which varies the flow rate of hydrocarbons to the intake manifold. Such flow rate may be controlled as function of engine RPM, temperature, manifold vacuum, etc. in cooperation with an electronic control unit of a known type.
It is a requirement of this type of system to control the hydrocarbon flow rate in an accurate and repeatable manner. To control the accuracy of the flow rate, it is necessary to control the magnitude and direction of the magnetic flux generated upon activation of the solenoid which is lodged within the valve's body.
Characteristic of the prior art are valves having cylindrical armatures movable between a position stop formed by a stator and a second stop in contact with a vent passage. The cylindrical armature in response to a control circuit is either modulated or controlled in an on/off manner to cause the armature to move between the first and second positions stops. Typically in order to reduce acoustical noise these armatures may include an elastomeric members at either end to reduce the noise resulting upon impact with the mechanical stops. However, even with the elastomeric members the valves are exceedingly acoustically noisy, such noise increasing as the operating temperature drops. In some instances the noise is sufficiently loud to be heard within the passenger compartment. An additional problem associated with the prior art is that such valve is typically non-linear as illustrated by curve A of FIG. 1. These valves display a significant non-linearity between 10-20 percent duty cycle. It is believed that this non-linearity is caused by the rebound behavior of the rubber elastomeric members on the armature as they impact the hard metal stator or the imposingly situated seat. Linearity of the operation is extremely important in that it determines how much of the flow rate curve can actually be used. As an example, when using a device such as that described above one would not be able to use a signal below a 20% duty cycle and acheive a repeatable and consistent behavior of the valve. In todays pollution control systems it is a requirement to more precisely control low canister purge flow rates in order to accomodate the stringent demands of vapor purge at engine idle conditions. As such, constant and predictable behavior and control is required to span the entire duty cycle range from 0-100% which requirement cannot be met by devices such as described above.
In view of the above it is an object of the present invention to provide an electromechanical valve having a broad operating range. A further object of the present invention is to provide a valve characterized by a low acoustic noise output level. An additional object of the invention is a valve which, within its magnetic circuit does not need a conventional stator.
Accordingly, the invention comprises: a flow rate valve comprising a non-magnetic housing comprising a central cavity and a pressure chamber, first and second ports extending into the pressure chamber. The interior end of the first port forms a valve seat. A bobbin is received within the cavity including a coil and a central blind bore enclosed at one end. The bobbin and housing include opposingly situated circumferential grooves. The valve further includes an armature assembly comprising a cylindrical armature movable within the bore, and a valve element movable with the armature between first and second positions, for opening and closing the valve seat, and means for limiting armature motion to prevent same from impacting the bottom of the bore in response to a magnetic field generated by the coil. The limiting means includes a resilient diaphragm comprising a circumferential lip received within the grooves, a first cylindrical portion about one end of the armature and a second cylindrical portion forming the valve element. The diaphragm in cooperation with the housing defining the pressure chamber.
Many other objects and purposes of the invention will be clear from the following detailed description of the drawings.